Heat generating systems with furnaces for combusting fossil fuels have long been employed to generate controlled heat, with the objective of doing useful work. The work might be in the form of direct work, as with kilns, or might be in the form of indirect work, as with steam generators for industrial or marine applications or for driving turbines that produce electric power. Modern water-tube furnaces for steam generation can be of various types including fluidized-bed boilers. While there are various types of fluidized-bed boilers, all operate on the principle that a gas is injected to fluidize solids prior to combustion in the reaction chamber.
In circulating fluidized-bed (CFB) type boilers a gas, e.g. air, is passed through a bed of solid particles to produce forces that tend to separate the particles from one another. As the gas flow is increased, a point is reached at which the forces on the particles are just sufficient to cause separation. The bed then becomes fluidized, with the gas cushion between the solids allowing the particles to move freely and giving the bed a liquid-like characteristic. The bulk density of the bed is relatively high at the time of fluidization, but will decrease as it flows upward through the reaction chamber where it is combusted to generate heat.
The solid particles forming the CFB typically include fuel particles, such as crushed or pulverized coal or other solid fuel, and sorbent particles, such as crushed or pulverized limestone, dolomite or other alkaline earth material. Combustion of the CFB in the reaction chamber of the boiler produces flue gas and ash. During the combustion process, carbon in the fuel is oxidized resulting in the generation of carbon dioxide (CO2). Nitrogen is also oxidized resulting in the generation of nitrogen oxide (NOx). Additionally, sulfur is oxidized to form sodium dioxide (SO2). The CO2, NOx, SO2 and other gasses generated during combustion form the flue gas. The ash consist primarily of unburned solids including inert material and sorbent particles. The ash, or some portion thereof, is sometimes referred to as particulate matter. The ash is entrained and carried in an upwardly flow by the hot flue gas, and is exhausted from the furnace with the hot flue gas. During this flow, the SO2 in the flue gas will be absorbed by the sorbent.
An air pollution control (APC) subsystem is conventionally used to remove various so called pollutants, including CO2, NOx, SO2 and particulate matter, from the flue gas produced by such heat generating systems. Thus, the flue gas exhausted from the furnace is directed to the various components of an APC subsystem before reaching the stack and being exhausted into the atmosphere. Each of the APC components can be considered a system in its own right. For example, the flue gas may be processed via cyclone separator and/or electrostatic precipitator to remove particulate matter, via a selective catalytic reduction (SCR) system to remove NOx, via a SO2 scrubber system to remove SO2, and via a CO2 scrubber system to remove CO2.
However, there are also other ways to reduce emissions. For example, it is known that CO2 and NOx emissions can be reduced by using oxygen in the combustion process. More particularly, U.S. Pat. No. 6,505,567, which issued on Jan. 14, 2003, is entitled “Oxygen Fired Circulating Fluidized Bed Steam Generator” and is assigned to the assignee of the present application, describes a CFB steam generating system that uses oxygen, in lieu of air, to fluidize the fuel in the CFB. The described system facilitates the use of CO2 both as a desirable end product and in support to the combustion process. The disclosure of the '567 patent is incorporated in its entirety herein by reference.
According to the described technique, a substantially pure oxygen feed stream is introduced into a CFB steam generator and the fuel is combusted in the presence of the substantially pure oxygen feed stream to produce a flue gas which has CO2 and water vapor as its two largest constituent elements by volume and which is substantially free of NOx. The flue gas is passed through an oxygen feed stream pre-heater, which transfers heat from the flue gas to the oxygen feed stream. Furthermore, the flue gas is separated into an end product portion and a recycling portion. The end product portion of the flue gas is cooled and compressed so as to yield CO2 in a liquid phase and the recycling portion of the flue gas is directed back to the CFB steam generator to contribute to the combustion process therein.
The technique disclosed in the '567 patent offers the flexibility to use the produced CO2 as a desirable end product and in support of the combustion process. The production of liquid CO2 also improves the heat output of the heat generating system. However, while the disclosed technique can be used to significantly reduce CO2 emissions, there remains a reluctance in many quarters to add coal fired heat generating system capacity because of concerns regarding the future governmental regulation CO2 emissions and the costs of meeting these regulations. In this regard, studies have shown that the investment costs to retrofit traditionally designed CFB coal fired steam generating system for CO2 capture can be in the range of $1000 to $1600 per kilowatt (kW). Studies have also shown that the energy penalty for CO2 capture can range from 25% to 40%. Furthermore, particularly in retrofit situations, the system site itself may be insufficient to accommodate an architecture of the type described in the '567 patent.
Thus, while there is a recognized need for more heat generating system capacity to, for example, produce additional electrical power, and it is also recognized that CFB coal fired systems are an efficient means to generate such heat, the ongoing debate over global warming, and the increasing attention being given to CO2 emissions from the burning of fossil fuels such as coal, and most particularly the cost of capturing CO2 in terms of both capital expense and reduced energy production, have undoubtedly delayed some if not many installations, which could increase capacity and thereby increase the availability of power to the nation and the world.
Accordingly, a need exist for a new technique for capturing CO2 generated by CFB fossil fuel fired steam generating systems.